1. Field of the Invention
The present invention relates to a thin film laminate and particularly to a luminescent element having the thin film laminate structure.
2. Background Art
A thin film laminate comprising a lipophilic material-containing thin film and a hydrophilic material-containing thin film stacked on top of each other has been developed. Specific examples of thin film laminates include luminescent elements, particularly electroluminescent elements (hereinafter referred to often as “EL elements”).
The EL element comprises a pair of opposed electrodes and a luminescent layer held between the opposed electrodes. Recombination of electrons introduced from one of the electrodes and holes introduced through the other electrode in the luminescent layer induces luminescence of the luminescent layer. In 1963, M. Pope, H. P. Kallmann et al. have found that the application of a DC voltage to a single crystal of anthracene causes luminescence. This finding has led to full-scale research and development of the above element, and an organic EL element utilizing an organic thin film laminate structure was reported for the first time by T. W. Tang et al., KODAK, in 1987. Thereafter, a large number of research and development have been carried out with a view to improving the function of organic EL elements.
General layer construction of organic EL elements is shown in FIG. 1. The organic EL element generally comprises organic layers, i.e., an electron transport layer, a hole transport layer, and a luminescent layer, which greatly affect luminescent characteristics and color characteristics. The electron transport layer is formed of, for example, anthraquiodimethane. The hole transport layer is formed of, for example, phthalocyanine. The luminescent layer is formed of, for example, pyrene.
Most of organic EL elements include a hole transport layer and an organic luminescent layer. Materials used for constituting the hole transport layer in the organic EL element formed by coating are generally hydrophilic. Due to this nature of the material, the hole transport layer is formed by coating a coating liquid, prepared by dissolving or dispersing the layer forming material in water, onto a substrate. An organic luminescent layer is formed on the surface of the hole transport layer. Materials for organic luminescent layer formation are generally lipophilic. The organic luminescent layer is generally formed by dissolving or dispersing the layer forming material in an organic solvent to prepare a coating liquid and coating the coating liquid.
The organic EL element is generally likely to be influenced by moisture, and the performance of the element is often easily deteriorated by moisture. To avoid the performance deterioration, in many cases, effort has been made to satisfactorily seal the element for eliminating the adverse effect of the moisture. However, the hole transport layer is formed from a water-containing coating liquid. Therefore, removing water fully from the hole transport layer without leaving even a very small amount of water has been regarded as being very difficult because of the use of the hydrophilic material for the hole transport layer formation. Against this, in order to suppress the adverse effect of the very small amount of residual water within the hole transport layer, for example, a method has been adopted in which, in sealing the element, a moisture absorbent or the like is incorporated in the EL element structure. This method, however, is complicated and further is high in cost.